Automobile manufacturers have been designing vehicles with radio systems for many years. A basic onboard radio system typically includes an AM/FM receiver and one or more antennas that are designed to receive RF signals in the AM and FM frequency bands. Older vehicles employ external whip antennas that tend to detract from the visual appearance of the vehicles. Such whip antennas are also prone to breakage and can adversely impact the aerodynamic performance of the vehicles.
Some modern vehicles incorporate an FM antenna element into the windshield glass and/or the rear window. Moreover, some vehicles utilize the rear window defroster element as the FM and/or AM antenna element. Such deployments, however, require sufficiently large windows that can adequately accommodate the antenna element(s). Unfortunately, certain vehicle designs, such as hatchbacks, compacts, and convertibles, lack the window space for integrated antennas. Moreover, integrating an AM antenna into the rear window defroster element requires large inductors in line to the defroster power and ground feeds, which increases cost, vehicle mass, and parts count, and introduces packaging issues. Accordingly, it may be difficult or impossible to deploy a window-integrated antenna in some vehicles.
Although a window-integrated antenna results in better aerodynamics and a cleaner overall vehicle appearance, the antenna element itself remains visible as a wire or conductive trace in the window. Thus, although less physically obtrusive, a window-integrated antenna remains visible to the customer. Such antenna visibility may be undesirable to some customers.
It is well established that AM frequencies are highly susceptible to RF interference and, in particular, RF interference caused by onboard sources such as the vehicle ignition system, motors, generators, switches, controllers, etc. A window-integrated AM antenna may have higher sensitivity to such interference, due to the manner in which it is applied to the interior surface of the window. Thus, a window-integrated AM antenna may perform less than adequately and/or it might require additional components or circuitry designed to filter noise received from onboard sources of RF energy.